In recent years, electric motor vehicles such as electric vehicles, electric bicycles, electric fork lifts, and electric wheelchairs; portable devices such as power tools and portable lighting devices; and portable electronic devices such as personal computers and portable telephones are demanded to have longer lives, higher outputs, smaller sizes, lighter weights, and lower prices. In order to meet such demands, secondary batteries for such devices are required to have still smaller size, higher performance, and lower prices.
At present, lead storage batteries and nickel-cadmium batteries are predominantly used as secondary batteries. In addition, nickel-hydrogen batteries and lithium ion batteries are being put into practical use. However, these batteries still have considerable problems to be solved in order to meet the above demands, and thus do not yet have satisfactory properties.
Specifically, for example, lead storage batteries are highly reliable and can be produced at low cost, but are insufficient in output density per weight and energy density. Nickel-cadmium batteries have been drawing attention for the problems in the treatment after use since they contain cadmium. In the environmental point of view, the use of nickel-cadmium batteries are anticipated to be limited. The newly developed nickel-hydrogen batteries have performance comparable to nickel-cadmium batteries, but have problems in its high cost associated with the hydrogen absorbing alloy. Lithium ion batteries are still costly due to its cathode material, electrolyte, and separator having shutdown function, and thus anticipated to have difficulty in wide use.
In view of the above, secondary batteries which can be produced at low cost, which are free from the risk of environmental pollution upon disposal, and which have high output density and high energy density have been demanded, and nickel-zinc batteries, air-zinc batteries, silver oxide-zinc batteries, and manganese oxide-zinc batteries have been developed for meeting these demands. However, these batteries have serious drawbacks in that arborescent zinc dendrite grow from the zinc electrode during charging, and finally reach the nickel electrode to cause short-circuiting inside the battery. Thus, such alkali-zinc batteries have not yet been put into practical use in a universal and industrial scale.
In order to prevent the short-circuit due to the zinc dendrite, there have been proposed to add an inhibitory agent to the zinc electrode or the electrolyte for suppressing the crystal growth, or to modify the battery structure with controlled amount of electrolyte. Nickel-zinc secondary batteries wherein such modification is combined with use of a cellophane separator have been developed and some of them have been used in practice. In these batteries, however, the growth of dendrite is not suppressed completely, and thus the risk of short-circuit due to penetration of the cellophane separator by dendrite has not been avoided, so that the cycle life of the batteries are still unreliable.
A different approach for preventing short-circuit inside the battery due to dendrite has been proposed, wherein the growth of dendrite is prevented by the separator (U.S. Pat. Nos. 4,154,912 and 4,272,470). This approach features cross-linking of polyvinyl alcohol (referred to as PVA hereinbelow) molecules in a PVA film by acetalization to form networks between the molecules, thereby delaying the arrival of zinc dendrite to the nickel electrode.
Films obtained by the above conventional methods have problems in that those having good properties to suppress growth of dendrite have high electrical resistance (film resistance) and thus are not suitable as battery materials, whereas those having reduced film resistance are easily penetrated by dendrite and thus do not offer improvement in battery life.
Further, when the cross-linking reaction of PVA including two successive reactions, i.e. oxidative cleavage of 1,2-diol units in PVA to form aldehydes and acetalization of two hydroxyl groups in PVA with aldehyde using the catalysis of proton, are effected in two separate steps, the film strength is lowered during the former oxidative cleavage step to cause difficulties in subsequent handling and treatment of the film. In addition, the property to suppress growth of dendrite may not be improved sufficiently in proportion to increase in the film resistance with the progress of cross-linking.
In assembling batteries in an industrial scale, the electrode parts including the retainers (a material capable of wicking electrolyte) interposed between the electrodes and the separator are usually assembled first, and then an appropriate amount of electrolyte is charged, whereby the separator is preferably swelled to saturation.
However, if the time required for the separator to swell to saturation in the electrolyte is too long, the production efficiency of the batteries is lowered. Further, when the swelling rate of the separator is remarkably slower than the swelling rate of the retainers, other parts of the battery such as the retainers will swell first in the limited space between the electrodes, so that there will be little room for the separator to swell sufficiently. As a result, the film resistance is increased and the battery properties are deteriorated. In addition, when the separator is expanded more in the two-dimensional direction in the electrolyte, the separator is likely to be puckered with the progress of swelling, thereby lowering the battery properties. The slower the swelling rate of the separator in the electrolyte, the more remarkable this tendency will be. In the light of these, it is preferred, in view of battery assembling, that the swelling rate of the separator in an alkali electrolyte is as fast as possible, and that the swelling of the separator in the two-dimensional direction is as small as possible.